In electronic, commutated electric motors (also called EC motors or BLDC motors) a three-phase network is generated across a converter circuit, passed to the coils of the stator of the electric motor and a rotating stator magnetic field thus generated. The rotor of the electric motor often has one or more permanent magnet(s) by which a static rotor magnetic field is generated. A torque, which sets the rotor in motion, results from the interaction of the stator magnetic field with the rotor magnetic field.
The converter circuit of the EC motor controls the phases of the three-phase network as a function of the position of the rotor which usually has to be determined metrologically. Sensors, such as Hall sensors, reed sensors or the like, which directly measure the rotor magnetic field, are frequently provided to determine the position, in particular to determine the angle of rotation, of the rotor. Alternatively, sensorless angle of rotation detectors are often used to determine the position of the rotor and not least of all for cost reasons. In this case what is known as the counter-electromotive force (counter-EMF) of the electric motor, i.e. the voltage induced in the stator coils by the rotating rotor magnetic field, is measured to determine the position. Motors of this kind are called brushless, sensorless electric motors.
A drawback of the sensorless angle of rotation detectors is that the counter-EMF can usually only be measured in a motor phase that is at zero current for the duration of the measurement. A number of measurement angles or measurement ranges is therefore provided, within which the relevant phase is kept at zero current, for one full cycle of the commutation (which with a 1-pole synchronous motor corresponds to a 360° full cycle of the rotor rotation) for measuring the counter-EMF. On the other hand the maximum size and position of the commutation angle is restricted by the or each measurement angle, the commutation angle in general clearly falling below the theoretically possible maximum angle of 180° as a result of this restriction. The commutation angle designates that part of the full cycle during which the or each phase is controlled in terms of circuitry, i.e. is excited. Commutation angles between 120° and 150° are usually used. An increase in the commutation angle usually leads to an increase in the output and efficiency of the electric motor, so it is often desirable to achieve the largest possible commutation angle.
The phases of an electric motor are conventionally symmetrical to each other, i.e. controlled in the same way. In particular commutation angles and measurement angles are adjusted in the same manner for all motor phases in this connection. Moreover, with bipolar commutation of an electric motor, the two commutation angles which correspond to positive and negative control of the same motor phase during the full cycle are also conventionally selected so as to be symmetrical, two identical measurement angles being alternately provided between the two commutation angles.
It is conventional to commutate the motor phases with the zero crossing of the counter-EMF in the corresponding motor phase. A positive advanced ignition can be used to obtain a greater output from a motor. In this connection it is not the zero crossing of the induced voltage that is detected as the trigger or start signal for the commutation angle, instead the counter EMF is compared with a reference voltage that differs from zero (in particular UL>0 volt with decreasing counter-EMF). The reference voltage must however be smaller than the intermediate circuit voltage since owing to the system the induced voltage is limited to the amount of the intermediate circuit voltage by the converter circuit. This limitation leads to limiting of the maximum adjustable advanced ignition angle and thus in turn to a limited output for the motor.